To a solution of tributylmethylammonium chloride (728 mg, 3.09 mmol) in DCM (10 ml) was added potassium permanganate (488 mg, 3.09 mmol) portionwise over 10 minutes at room temperature. The reaction mixture was allowed to stir under an atmosphere of nitrogen for 30 minutes. The mixture was cooled down to 0° C. and treated dropwise with a solution of tert-butyl 2,3-diphenylpyrido[2,3-b]pyrazine-5(6H)-carboxylate (step 1)(700 mg, 1.816 mmol) in DCM (8 ml). The reaction was then left to stir for a further 2 hours at 0-5° C. under an atmosphere of nitrogen. A solution of sodium bisulfite (1134 mg, 10.90 mmol) in water (9 ml) was added dropwise at 0-5° C. to the reaction mixture. The mixture was filtered through Celite® (filter material) and washed with DCM (20 ml) and water (10 ml). The organic layer was separated and concentrated in vacuo to give a foamy solid. The crude material was purified by chromatography on silica eluting with 0-90% EtOAc/iso-hexane to afford the title compound;
